Materials Science Quiz: Aluminum and Steel

Metal Packaging

• Metallic elements have a compact chemical structure, resisting diffusion.
• They are malleable, allowing for various shapes.
• High thermal conductivity facilitates heat transfer.
• High electrical conductivity allows for electricity.
• Metal alloys are often used in food packaging, primarily based on aluminum, tinned steel, stainless steel, copper, and nickel. 
• Aluminum alloys are commonly coated with tin, chromium oxides, or varnishes.
• Stainless steel is used for food contact applications (containers, vessels, utensils).
• Copper and cast iron are occasionally used for food contact purposes (e.g., dairy boilers, grill tools).
• Nickel is used as a sandwich layer between steel and tin.
• Lead is occasionally used for soldering.
• Copper is used for welding can seams.

Metallic Bonds

• Metallic bonds are responsible for the strength, malleability, and ductility of metals.
• Hammering or stretching can cause metal layers to slide, allowing the free electrons to re-arrange without disrupting the bond.
• Metallic bonds are responsible for high electrical and thermal conductivity, since the free electrons can move throughout the lattice of metal ions.

Distinct Advantages of Metal Packaging over Glass

• Good heat transmission, facilitating rapid heating and cooling. 
• Lighter weight compared to glass.
• Resistant to breaking and physical damage.
• Impermeable to both light and air.

Container Designs

• The images show typical round-section food and beverage cans.
• The images show example handles and designs.

Steel (Raw Materials)

• Steel is a wide family of iron alloys with a low carbon content.
• Carbon acts as a binding agent.
• Higher carbon content correlates to higher tensile strength and brittleness.
• Low-carbon steel is frequently used in containers.
• Steel is typically coated to prevent interaction of the metals with food products.

Tinplate

• Tinplate is low-carbon steel coated with a thin layer of tin, improving corrosion resistance making it suitable for direct food contact.
• Organic coatings often coat the inside of tinplate, preventing interactions between the metal and food product.

Tin-Free Steels

• Tin-free steels are utilized in cases where tin is not needed or is cost-prohibitive.
• Cold-rolled steel coated with electrolytic chromium or chromium oxide is a desirable alternative.
• They offer resistance to heat, lacquer adhesion, and printing.
• Tin-free steels might be less resistant to acidic environments.

Polymer Coated Steels

• Polymer coatings are applied to tinplate or tin-free steels for better appearance, corrosion resistance and moisture barriers.
• Thermoplastic polymers are often used in such coatings.

Stainless Steel (Raw Materials)

• Stainless steel is an iron alloy with enhanced corrosion resistance, stemming from the high chromium content (minimum 11%).
• Chromium reacts with oxygen forming a protective chromium oxide layer.
• The uniform composition across the can material avoids needing further protective coating.
• Stainless steel is divided into three basic groups (Austenitic, Ferritic, and Martensitic).
• Austentic is important for food applications.

Aluminum (Raw Materials)

• Aluminum is a very abundant metal in the Earth's crust.
• It's a light-weight material with high energy requirements for production.
• Electrochemically, aluminum reacts readily with oxygen, forming a thin passivated oxide layer, thus protecting itself from corrosion.

Aluminum Properties

• Aluminum alloys' strength and ductility are maintained across temperature variations.
• High malleability and workability facilitate shaping and forming the metal.

Aluminum Drawbacks

• Aluminum is very expensive to manufacture and weld.
• It is less resistant to corrosion than steel.

Aluminum Foil

• Aluminum foil is made through rolling a sheet of aluminum alloy.
• It has excellent water, gas, and aroma barriers.
• It's lightweight and resistant to many foods, but reacts somewhat with acidic and/or salty foods. 
• Its dead-fold property can lead to pinholes during handling.

Recycled Packaging Metal

• Both aluminum and steel-based materials can be re-melted and re-used, with no loss of quality, during the re-melting process.

Three-Piece Welded Cans

• These cans consist of a body blank and two ends.
• Steel is the primary material used in this type of manufacturing.
• Aluminum generally is not suitable for welding in this process.

Two-Piece Single-Drawn and Multiple-Drawn Cans

• Pre-coated, laminated, and printed tinplate or TFS is typically used in this process.
• The cans are drawn into different shapes.

Two-Piece Drawn and Wall Ironed (DWI) Cans

• Typically using uncoated tinplate, these cans undergo a heat process, making aluminum unsuitable.
• Specific steps are undertaken to form the end shapes and draw the cans into various shapes.

End-Making Process

• This process prepares and finishes the ends of the cans.
• Steps include preparing the end shells, converting them into easy-open ends, etc.

Coatings, Film Laminates, Inks

• Organic materials like coatings, films, inks, etc. are applied to create barriers and enhance the appearance of both the container and components.
• Coating and decoration are applied to different types of containers (e.g., 2- and 3-piece cans) prior to the forming operation.
• Coating application might be based on the form (i.e. flat, coil, or sheet), thus the processes differ. 

Can Reception at Packers

• Inspection in the packing stage determines things such as dimensions, seam defects, presence, or lack of internal lacquer, presence of defects, quality, etc.

Filling and Exhausting

• Accurate and consistent filling is critical, avoiding damage to the can and contamination.
• Avoiding external contamination and maintaining an appropriate headspace (oxygen levels) in cans reduces the chances of corrosion. 

Seaming

• Double-seam formation involves two separate processes on the can end and body.
• Key parameters, such as seam thickness, height, and overlap, are measured and tracked.

Heat Processing

• Sterilization typically ranges from 115°C to 135°C.
• Pasteurization occurs within the range of 90°C to 105°C. 
• Temperature differences between the process and the can interior can lead to distortion issues.

Post-process

• Proper cooling using water prevents bacterial risks (e.g., botulism spores) and promotes rusting prevention.
• Proper drying prevents microbial growth and corrosion.

Container Handling

• Any defect on the container needs to be regarded as a critical concern.
• Seam areas are prone to contamination.

Storage and Distribution

• Temperatures and humidity levels must be managed to prevent condensation and further corrosion.
• Freezing conditions need to be avoided to avoid expanding issues.
• Physical handling needs to be careful.

Shelf Life of Canned Foods

• Canning method is crucial to food stability.
• Possible chemical reactions, color, flavor, and interactions with the containers can influence the shelf life of foods.
• Minimum durability refers to the time a product remains marketable during normal storage.
• Technical shelf life is the time a product remains fit to eat during normal usage.
• Different factors influence shelf life, including sensory quality (color, flavor), nutritional stability, and container/product interactions.

Corrosion

• Corrosion in metals typically involves two steps — an initial ionization by metal and subsequent electron transport through the material.
• Oxygen and other oxidizers are common corrosion facilitators.
• The resulting metal oxides are a key component in corrosion and rust formation.

Interaction between Can and Contents

• Tin protects the steel base-plate from corrosion.
• Lacquering is common to treat or prevent interactions between containers and contents, and in many cases, prevents dissolution of certain metals into food products.

Internal Corrosion

• This can emanate from mechanical damage as well as specific reactions between can and content.
• Inadequate lacquers or cracking in lacquers can significantly increase the chances of internal corrosion.

Stress Corrosion Cracking

• Stress corrosion cracking, when a particular environment allows for a more rapid corrosion, most commonly due to external stresses in metals, internal stresses from cold working, and certain types of metals.
• Determining the starting points for cracks can be very difficult—this process is most common in the beaded part of canned metal containers.

Environmental Stress Cracking Corrosion of Aluminum Alloy Beverage Can Ends

• This type of corrosion can develop in specific situations in can ends. 
• Such issues can appear at pull-tab or stay-on-tab ends.
• Handling and drying containers before storage can reduce the risks of contamination. 

Sulfur Staining

• Sulfur compounds from food products can react with the can metal
• A reaction that leads to sulfide stains.

External Corrosion

• Rusting is a type of external corrosion.
• Corrosion occurs due to condensation, temperature fluctuations, and the presence of moisture, oxygen, and certain metal elements.